Re: [PATCH] mm/gup: fix GUP-fast fallback for NULL-mapping order-0 folios

[David Hildenbrand (Arm)]

On 4/9/26 18:55, Zi Yan wrote:
> On 9 Apr 2026, at 11:11, David Hildenbrand (Arm) wrote:
> 
>> On 4/9/26 17:05, Zi Yan wrote:
>>>
>>>
>>> The idea is to give every allocated page a NotRmappable page_type and
>>> 1) let page_rmappable_folio() remove the page_type by setting mapcount
>>> to 0, since page_type overlaps with mapcount;
>>> 2) allow callers of alloc_page*() to change page_type from NotRmappable
>>> to other types.
>>
>> Could we instead just set the special type in vm_insert_page(), after
>> verifying that it does not have any other type?
> 
> You mean in validate_page_before_insert() we set the page to NotRmappable
> if it can be inserted, then in the rmap code we reject NotRmappable
> page/folios? And NotRmappable will remain until the page/folio is freed.
> That should work. So the purpose is to avoid setting the page_type if
> a page/folio is never going through vm_insert_page()?

Yes, only these non-folio pages will show up process page tables.

We would add a safety check that the mapcount really is 0 (-1) or
already the proper type (NotRmappable).

Rmap code would then simply skip any such thing.

-- 
Cheers,

David